Display device and method for driving the same

ABSTRACT

A display device displaying a color by mixing light reflected by a first reflection element  22  and light reflected by a second reflection element  26  by additive color mixture, in which the light of a first wavelength reflected by the first reflection element and light of a second wavelength reflected by the second reflection element have a mutually complementary color relationship. Thus, the display device, which can make good black and white display by a simple structure and can be driven by a simple method, can be realized.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is a Continuation of International ApplicationNo. PCT/JP02/08554, with an international filing date of Aug. 26, 2002,which designating the United States of America.

TECHNICAL FIELD

[0002] The present invention relates to a display device, morespecifically a reflective display device, and a method for driving thesame.

BACKGROUND ART

[0003] Generally, CRTs, transmittive liquid crystal displays withbacklights are generally used in the display devices of computers andmobile devices. The displays of this type are the so-called emissivedisplays which include internal emission means.

[0004] Based on recent studies, it is proposed to preferably usenon-emissive reflective display devices in terms of work efficiency andfatigue in reading texts, etc., on display. The reflective displaydevice, which requires no internal emission means and uses naturallight, etc. for display, is good for the eye and effective to decreasethe electric power consumption.

[0005] To realize further lower electric power consumption, displaydevices having the memorization ability to retain displayed informationeven when their source power is turned off are expected.

[0006] As such display devices are proposed display devices using chiralnematic liquid crystals and cholesteric liquid crystals. Chiral nematicliquid crystals are liquid crystals comprising nematic liquid crystalsand chiral catalysts added to the nematic liquid crystals. Chiralnematic liquid crystals and cholesteric liquid crystals have acharacteristic of reflecting selectively light of specific wavelengths.

[0007] A proposed display device using a chiral nematic liquid crystalwill be explained with reference to FIG. 35. FIG. 35 is a schematic viewof the proposed display device using a chiral nematic liquid crystal.

[0008] As shown in FIG. 35, a photo-absorbing layer 114 is formed on asubstrate 110 of glass. An electrode 112 of ITO (Indium-Tin-Oxide) isformed on the photo-absorbing layer 114. A substrate 118 of glass isformed on the substrate 110 with the photo-absorbing layer 114 and theelectrode 112 formed on, opposed to the substrate 110. An electrode 120of ITO is formed on the side of the substrate 110, which is opposed tothe electrode 112. A liquid crystal layer 122 of chiral nematic liquidcrystal is provided between the substrate 110 with the photo-absorbinglayer 114 and the electrode 112 formed on and the substrate 118 with theelectrode 120 formed on. Thus, the display device using the chiralnematic liquid crystal is constituted.

[0009] A display device using a chiral nemtaic liquid crystal isdisclosed in, e.g., Japanese published unexamined patent application No.Hei 06-507505.

[0010] Then, the operation of the display device using the chiralnematic liquid crystal will be explained.

[0011]FIG. 35A shows the planer state. In the planer state, that of theincident light, whose wavelength corresponds to a helical pitch of theliquid crystal molecules is reflected. A wavelength λ for a maximum on areflection spectrum is expressed by

λ=n·p

[0012] wherein an average refractive index of the liquid crystal isrepresented by n, and a helical pitch of the liquid crystal isrepresented by p. Wavelength band width Δλ of reflected light isexpressed by

Δλ=Δn·p

[0013] wherein an isotropy of refractive index of liquid crystal isrepresented by Δn.

[0014]FIG. 35B shows the focalconic state. In the focalconic state, theincident light passes through the liquid crystal layer 122 of the chiralnematic liquid crystal and is absorbed by the photo-absorbing layer 114formed on the substrate 110. Accordingly, in the focalconic state, blackcolor is displayed.

[0015]FIG. 36 is a graph of reflection spectra of the chiral nematicliquid crystal. The wavelengths are taken on the horizontal axis, and onthe vertical axis reflectances are taken. The reflectances on thevertical axis were given when the reflection on a white reflection boardis 100%.

[0016] The reflection wavelength of chiral nematic liquid crystals canbe set at a prescribed value by suitably setting chiral catalyst amountsto be added to the cholesteric liquid crystals. The addition of largeramounts of chiral catalysts decreases the helical pitches p of theliquid crystals and shortens the wavelengths λ of the reflected light.

[0017] Chiral nematic liquid crystals have a characteristic ofreflecting either of right circularly polarized light and leftcircularly polarized light. This is described in, e.g., SID 97 DIGEST,p. 1019-1022. Characteristics of the chiral catalysts to be added to thechiral nematic liquid crystals can set the chiral nematic liquidcrystals to be right circularly polarized light or left circularlypolarized light. Chiral nematic liquid crystals, which reflect either ofthe right circularly polarized light and the left circularly polarizedlight, theoretically has the upper limit of the reflectance of 50%.

[0018] Planer state and focalconic state are retained substantiallypermanently unless an external force is applied to. Accordingly, the useof chiral nematic liquid crystals can provide display devices havingmemorization ability which can retain displayed information even whentheir power sources are turned off.

[0019] As described above, chiral nematic liquid crystals, which canconstitute reflective display devices and can retain displayedinformation even when the power sources are turned off, is noted asliquid crystals which will form the next generation display devices.

[0020] In a display device using a single layer of a chiral nematicliquid crystal, in the planer state, light of a wavelength correspondingto the helical pitch is selectively reflected, whereby the displaycolors are chromatic. On the other hand, in the focalconic state, theincident light is absorbed by the photo-absorbing layer, whereby thedisplay color is black. Accordingly, the display device using the singlelayer of the chiral nematic liquid crystal can display chromatic colorsor black color but cannot display white color.

[0021] Techniques of displaying white color by chiral nematic liquidcrystals are proposed as follows.

[0022] Japanese published unexamined patent application No. Hei09-503873 discloses the technique of mixing a plurality of kinds ofchiral nematic liquid crystals, whereby all the visible spectra of400-700 nm is covered to thereby display while color.

[0023] Japanese published unexamined patent application No. 2001-066627discloses the technique of providing four liquid crystals of R (red), G(green), B (blue) and Y (yellow), whereby substantially all the visiblespectra are covered to thereby display white color.

[0024] Japanese published unexamined patent application No. 2001-109012discloses that chiral nematic liquid crystals of three colors, RGB areused to display white color.

[0025] Japanese published unexamined patent application No. Hei11-231339 discloses that a chiral nematic liquid crystal layer whichreflects selectively light of yellow color is formed on aphoto-absorbing layer which absorbs light of blue color to display whitecolor.

[0026] Techniques of display white color by using light scattering infocalconic state are also proposed.

[0027] However, the reflection wavelength band of chiral nematic liquidcrystals has a full width at half maximum of about 70-110 nm. Thedisplay device disclosed in Japanese published unexamined patentapplication No. Hei 09-503873 cannot cover all the visible spectra onlyby mixing chiral nematic liquid crystals of, e.g., two kinds andaccordingly cannot display white color. Furthermore, such displaydevice, in which two or more kinds of liquid crystals are mixed in onepolymer, has a risk that the liquid crystals might be mixed with oneanother.

[0028] The display devices disclosed in Japanese published unexaminedpatent application No. 2001-066627 and Japanese published unexaminedpatent application No. 2001-109012 both requires three or more liquidcrystal layers, which is a blocking factor for the cost reduction. Thesedisplay devices have high drive voltages, and their drive methods arecomplicated.

[0029] The display device disclosed in Japanese published unexaminedpatent application No. Hei 11-231339 has blue and white display colors.Accordingly, the visibility of the display devices is low unsuitable toread documents, such as texts, etc.

[0030] The technique of displaying white color by the scattering oflight in focalconic state has the reflectance which is as low as about20%, and bright white color display cannot be obtained. Accordingly, thedisplay device using such technique cannot have high contrast.

[0031] As described above, none of the proposed techniques have beenable to provide inexpensive display devices having good white and blackdisplays.

[0032] Chiral nematic liquid crystals have a characteristic that as theobservation angle is increased, the selective reflection wavelengthsshift to the side of shorter wavelengths. Accordingly, in displaydevices using simply chiral nematic liquid crystals, hues of the displaycolors change depending on observation direction changes. FIG. 37 is aconceptual view of the observation angle change. For example, a displaywhich is in red when observed at the front changes to green as theobservation angle θ is increased, and changes to blue as the observationangle θ is further increased. For example, a display which is in greenwhen observed at the front changes to blue as the observation angle θ isincreased. Monitor display devices are required to have a ±60°visibility range. The hue change in the ±60° orange is not preferable.Accordingly, techniques of reducing the hue changes corresponding to theobservation angle changes have been expected.

DISCLOSURE OF INVENTION

[0033] An object of the present invention is to provide an inexpensivereflective display device which can make good black and white displaysand has little hue change corresponding to the observation directionchanges. Another object of the present invention is to provide a methodfor driving the display device, which is simple and can realize gooddisplays.

[0034] The above-described object is attained by a display devicedisplaying a color by mixing light reflected by a first reflectionelement and light reflected by a second reflection element by additivecolor mixture, in which the light having a first wavelength reflected bythe first reflection element, and the light having a second wavelengthreflected by the second reflection element have substantially mutuallycomplementary color relationship.

[0035] The above-described object is also attained by a method fordriving a display device including a first reflection element having aselective reflection wavelength in a 480-500 nm range and a secondreflection element having a selective reflection wavelength in a 580-640nm range, and displaying a color by mixing light reflected by the firstreflection element and light reflected by the second reflection elementby additive color mixture, in which a display state of the firstreflection element and a display state of the second reflection elementbeing both changed to switch between a white color display and a blackcolor display.

[0036] The above-described object is also attained by a method fordriving a display device including a first reflection element having aselective reflection wavelength in a 450-480 nm range and a secondreflection element having a selective reflection wavelength in a 570-610nm range, and displaying a color by mixing light reflected by the firstreflection element and light reflected by the second reflection elementby additive color mixture, in which a display state of the firstreflection element being fixed, and a display state of the secondreflection element being changed to switch between a white color displayand a blue color display or between a yellow color display and a blackcolor display.

[0037] According to the present invention, reflection lights reflectedon two liquid crystal layers mutually have a complementary colorrelationship, whereby a display device which can realize good black andwhite display can be provided.

[0038] According to the present invention, the selective reflectionwavelength of one of the liquid crystal layers is in a 480-500 nm range,and the other of the liquid crystal layers in a 580-640 nm range,whereby changes of the selective reflection wavelengths generated due toobservation angle changes can be compensated. A display device whichmakes no substantial change in hues of the display colors even when theobservation directions are changed can be provided.

[0039] According to the present invention, good black and white displaycan be realized by providing only two liquid crystal layers, whereby aninexpensive display device can be provided.

[0040] According to the present invention, the threshold voltages of therespective liquid crystal layers can be made substantially even, wherebya display device can have homogeneous display quality and much improvedstability in the drive.

[0041] According to the present invention, the selective reflectionwavelength of one of the liquid crystal layers is in a 450-480 nm range,and the selective reflection wavelength of the other of the liquidcrystal layers is in a 470-610 nm range, whereby one alone of the liquidcrystal layers is driven, whereby a display device which can makewhite-blue color display or yellow-black color display can be realized.Thus, the display device can have a simple structure and can be drivenby a simple method.

[0042] According to the present invention, chiral nematic liquidcrystals are used, whereby displayed information can be retained evenwhen the electric power is turned off. Accordingly, the presentinvention can provide a display device whose electric power consumptionis low and has memorization ability.

[0043] According to the present invention, one of the liquid crystallayers uses the R liquid crystal, which reflects right circularlypolarized light, and the other liquid crystal layer uses the L liquidcrystal, which reflects right circularly polarized light. Accordingly,even when a wavelength bans in which a reflection spectrum of one liquidcrystal layer and a reflection spectrum of the other liquid crystallayer overlap each other present, light of a wavelength band which isreflected on one liquid crystal layer can be prevented from beingreflected on the other liquid crystal layer. Thus, according to thepresent invention, when a reflection spectrum of one liquid crystallayer and a reflection spectrum of the other liquid crystal layeroverlap each other, the decrease of the light reflection on either ofthe liquid crystal layers can be prevented, whereby the luminosity ofthe white color display can be increased.

[0044] According to the present invention, the liquid crystal layer ofthe R liquid crystal and the liquid crystal layer of the L liquidcrystal are formed for the respective selective reflection wavelengthsλ₁, λ₂, whereby both the right circularly polarized light and the leftcircularly polarized light can be reflected. Accordingly, the presentinvention can provide a display device which can reflect the incidentlight with high efficiency and can provide white color display of higherluminosity.

[0045] According to the present invention, the substrate and thepartition layer are formed of film, whereby a display device which isflexible and can be used in extensive purpose.

[0046] According to the present invention, the thickness of thepartition layer, etc. are so set that a phase difference betweenordinary rays and extraordinary rays entering the liquid crystal layersis odd times λ₂/2, which permits good displays to be made even whenfilms having birefringence are used.

[0047] According to the present invention, the thickness of thepartition layer is so set that a phase difference between ordinary raysand extraordinary rays entering the liquid crystal layers is odd timesλ₂/2, which permits good displays to be made even when the R liquidcrystal and the L liquid crystal are combined.

[0048] According to the present invention, the liquid crystal layers aremicro-capsuled, whereby chiral nematic liquid crystals are preventedfrom mixing with each other even without the use of a partition layer.According to the present invention, it is not necessary to use apartition layer, which permits a display device to be thinned.

BRIEF DESCRIPTION OF DRAWINGS

[0049]FIG. 1 is an xy chromaticity diagram explaining the principle ofthe present invention (Part 1).

[0050]FIG. 2 is an xy chromaticity diagram explaining the principle ofthe present invention (Part 2).

[0051]FIG. 3 is a sectional view of the display device according to afirst embodiment of the present invention, which shows a structurethereof.

[0052]FIG. 4 is a graph of reflection spectra of the white color displayof the display device according to the first embodiment of the presentinvention.

[0053]FIG. 5 is a sectional view of the display device according to asecond embodiment of the present invention, which shows a structurethereof.

[0054]FIG. 6 is a graph of reflection spectra of the white color displayof the display device according to the second embodiment of the presentinvention.

[0055]FIG. 7 is a sectional view of the display device according to onemodification of the second embodiment of the present invention, whichshows a structure thereof.

[0056]FIG. 8 is a sectional view of the display device according to athird embodiment of the present invention, which shows a structurethereof.

[0057]FIG. 9 is a sectional view of the display device according to onemodification of the third embodiment of the present invention, whichshows a structure thereof.

[0058]FIG. 10 is a sectional view of the display device according to afourth embodiment of the present invention, which shows a structurethereof.

[0059] FIG.11 is a sectional view of the display device according to afifth embodiment of the present invention, which shows a structurethereof.

[0060]FIG. 12 is a circuit diagram of an equivalent circuit of a liquidcrystal cell.

[0061]FIG. 13 is a view of relationships between an applied pulse and avoltage applied to the liquid crystal layers.

[0062]FIG. 14 is views of relationships between the resistivity of theliquid crystal and a voltage applied to the liquid crystal layers.

[0063]FIG. 15 is views of state changes of the liquid crystal layersgiven when the threshold voltages of the liquid crystal layers aredifferent from each other.

[0064]FIG. 16 is views of state changes of the liquid crystal layersgiven when the threshold voltages of the liquid crystal layers aresubstantially equal to each other.

[0065]FIG. 17 is graphs of the response characteristics of the liquidcrystal layers given when the threshold voltage difference between theliquid crystal layers is large and small.

[0066]FIG. 18 is graphs of the voltage characteristics of the respectiveliquid crystal layers containing no additive.

[0067]FIG. 19 is a circuit diagram used in measuring the partial voltageratios of the respective liquid crystal layers.

[0068]FIG. 20 is graphs of the partial voltage ratios of the respectiveliquid crystal layers measured with ac pulses applied.

[0069]FIG. 21 is a graph of the measured voltage response of the displaydevice according to the fifth embodiment of the present invention.

[0070]FIG. 22 is a view explaining the structure and the operation ofthe photoconductive layer.

[0071]FIG. 23 is graphs showing the principle of the optical writingmethod using the photoconductive layer.

[0072]FIG. 24 is a sectional view of the display device according to asixth embodiment of the present invention, which shows a structurethereof.

[0073]FIG. 25 is a sectional view of the display device according to aseventh embodiment of the present invention, which shows a structurethereof.

[0074]FIG. 26 is a graph of a spectral luminous efficacy curve of theeye of man.

[0075]FIG. 27 is a graph of relationships between the width of thereflection band of the blue color layer and the luminosity of bluecolor.

[0076]FIG. 28 is a graph of relationships between the width of thereflection band of the blue color layer and the luminosity of whitecolor.

[0077]FIG. 29 is a graph of relationships between the width of thereflection band of the blue color layer and the contrast.

[0078]FIG. 30 is a graph of reflection spectra of the white color of thedisplay device according to the seventh embodiment of the presentinvention.

[0079]FIG. 31 is a graph of reflection spectra of the yellow color ofthe display device according to the seventh embodiment of the presentinvention.

[0080]FIG. 32 is a sectional view of the display device according to aneighth embodiment of the present invention, which shows a structurethereof.

[0081]FIG. 33 is a sectional view of the display device according to aninth embodiment of the present invention, which shows a structurethereof.

[0082]FIG. 34 is a view of one example of suitable ratio of partialvoltages applied to the respective layers given when a voltage isapplied between the electrodes.

[0083]FIG. 35 is diagrammatic views of the proposed display device usinga chiral nematic liquid crystal.

[0084]FIG. 36 is a graph of reflection spectra of the chiral nematicliquid crystal.

[0085]FIG. 37 is a schematic view as watched in different observationdirections.

BEST MODES FOR CARRYING OUT THE INVNEION

[0086] The Principle of the Present Invention The principle of thepresent invention will be explained with reference to FIGS. 1 and 2.FIGS. 1 and 2 are the xy chromaticity diagrams showing the principle ofthe present invention.

[0087] The inventors of the present invention have made earnest studiesand go an idea that two kinds of chiral nematic liquid crystals one ofwhich has a 480-500 nm selective reflection wavelength and the other ofwhich has a 580-640 nm selective reflection wavelength are combined sothat colors of light corresponding to the selective reflectionwavelengths are complimentary colors to each other, whereby good blackand while displays can be realized, and the hue changes of the displaycolors due to observation direction changes can be prevented.

[0088] A color produced by mixing 2 kinds of color light by additivecolor mixture is a color corresponding to the central coordinates of aline segment between two chromaticity coordinates in the xy chromaticitydiagram. When a selective reflection wavelength λ₁ of one chiral nematicliquid crystal is, e.g. 490 nm, and a selective reflection wavelength λ₂of the other chiral nematic liquid crystal is, e.g., 600 nm, the centralcoordinates of the line segment interconnection the two chromaticitycoordinates is coordinates corresponding to while color. Accordingly,two kinds of chiral nematic liquid crystals colors of the lightcorresponding to selective reflection wavelengths of which have thecomplementary color relationship with each other are combined, wherebygood while color can be displayed.

[0089] As shown in FIG. 2, as an observation angle is increased,selective reflection wavelengths λ₁, λ₂ of the chiral nematic liquidcrystals respectively shift to the shorter wavelength side. Theselective reflection wavelengths λ₁, λ₂ of the chiral nematic liquidcrystals both shift to the shorter wavelength side, and the centralcoordinates of the line segment interconnecting the two chromaticitycoordinates in the xy chromaticity diagram do not substantially change.Accordingly, even when the selective reflection wavelengths λ₁, λ₂ ofthe chiral nematic liquid crystals respectively shift to the shorterwavelength side, the influences of the changes of the selectivereflection wavelengths λ₁, λ₂ on the display color are mutuallycompensated, and the hue of the display color given by the additivecolor mixture make no substantial change. Thus, chiral nematic liquidcrystals having such selective reflection wavelengths λ₁, λ₂ arecombined, whereby a display device hues of the display colors make nosubstantial change with changes of the observation directions can beprovided.

[0090] In the above, the selective reflection wavelength λ₁ of onechiral nematic liquid crystal is 490 nm, and the selective reflectionwavelength λ₂ of the other chiral nematic liquid crystal is 600 nm, butthe combination of the selective reflection wavelengths of the chiralnematic liquid crystals is not essentially limited to the above. Theselective reflection wavelength λ₁ of one chiral nematic liquid crystalis within a range of 480-500 nm, and the selective reflection wavelengthλ₂ of the other chiral nematic liquid crystal is within a range of580-640 nm. Furthermore, these two kinds of chiral nematic liquidcrystals are combined so that colors of the light corresponding to theseselective reflection wavelengths λ₁, λ₂ have the complementary colorrelationship. The selective reflection wavelength λ₁ of one chiralnematic liquid crystal must be within a range of 480-500 nm, and theselective reflection wavelength λ₂ of the other chiral nematic liquidcrystal must be within a range of 580-640 nm, because unless theselective reflection wavelengths λ₁, λ₂ are within these ranges, thecentral coordinates of the line segment interconnection the twochromaticity coordinates cannot be coordinates corresponding to whitecolor, and the display color given by the additive color mixture cannotbe white color. Even if a display color near to white color is obtained,changes of the selective reflection wavelengths λ₁, λ₂ of the chiralnematic liquid crystals caused by observation direction changes cannotbe mutually compensated, and the hue of the display color changes withobservation direction changes.

[0091] As described above, according to the present invention, two kindsof chiral nematic liquid crystals whose reflection light mutually havethe complementary color relationship are combined, whereby a displaydevice which can display good white color can be provided. Furthermore,according to the present invention, the selective reflection wavelengthof one chiral nematic liquid crystal is within a range of 80-500 nm, andthe selective reflection wavelength of the other chiral nematic crystalis within a range of 580-640 nm, whereby changes of the selectivereflection wavelengths of the chiral nematic liquid crystals due toobservation direction changes can be compensated, whereby a displaydevice whose hues of display colors make no substantial change even withobservation direction changes can be provided. Furthermore, according tothe present invention, only two kinds of chiral nematic liquids are usedto display good black and white displays, whereby an inexpensive displaydevice can be provided.

[0092] [A First Embodiment]

[0093] The display device according to a first embodiment of the presentinvention will be explained with reference to FIG. 3. FIG. 3 is asectional view of the display device according to the presentembodiment, which shows the structure thereof.

[0094] As shown in FIG. 3, an electrode 12 of a 0.1 μm-thick ITO isformed on a substrate 10 of glass. A photo-absorbing layer 14 of a 1μm-thick is formed on the electrode 12. A 30 nm-thick partition layer 16of glass is formed over the electrode 10, opposed to the substrate 10. Asubstrate 18 of glass is formed over the partition layer 16, opposed tothe partition layer 16. An electrode 20 of ITO is formed on the side ofthe substrate 18, which is opposed to the partition layer 16.

[0095] For example, a 5 μm-thick liquid crystal layer 22 of a chiralnematic liquid crystal having a selective reflection wavelength λ₁ of495 nm is formed between the substrate 18 with the electrode 20 formedon and the partition layer 16. The liquid crystal layer 22 is sealedwith a sealing compound 24. The liquid crystal molecules of the chiralnematic liquid crystal are twisted right. The chiral nematic liquidcrystal (hereinafter temporarily called “R liquid crystal”) whose liquidcrystal molecules are twisted right reflects only right circularlypolarized light.

[0096] The chiral nematic liquid crystal forming the liquid crystallayer 22 can be formed by adding a chiral catalyst to a nematic liquidcrystal. The nematic liquid crystal can be, e.g., E48 from Merck KGaA.The chiral catalyst can be, e.g., CB15 from Merck KGaA. This chiralcatalyst has a characteristic of inducing liquid crystal molecules totwist right. The selective reflection wavelength λ₁ of the chiralnematic liquid crystal can be suitably set by adjusting the amount ofthe chiral catalyst to be added to the nematic liquid crystal.

[0097] A liquid crystal layer 26 of a chiral nematic liquid crystal of,e.g., a 5 μm-thick and a 601 nm selective reflection wavelength λ₂ isformed between the substrate 10 with the electrode 12 and thephoto-absorbing layer 14 are formed and the partition layer 16. Theliquid crystal layer 26 is sealed with a seal compound 28. The liquidcrystal layer 26 is formed of the R liquid crystal. That is, the liquidcrystal molecules of the chiral nematic liquid crystal forming theliquid crystal layer 26 are twisted right.

[0098] The chiral nematic liquid crystal forming the liquid crystallayer 26 can be formed by adding a chiral catalyst to a nematic liquidcrystal, as described above. The nematic liquid crystal can be, e.g.,E48 from Merck KGaA, as described above. The chiral catalyst can be,e.g., CB15 from Merck KGaA, as described above. The selective reflectionwavelength λ₂ of the chiral nematic liquid crystal can be suitably setby adjusting the amount of the chiral catalyst to be added to thenematic liquid crystal.

[0099] The liquid crystal display according to the present embodiment,which includes the liquid crystal layer 22 having the selectivereflection wavelength λ₁ on the side of the observation, and the liquidcrystal layer 26 having the selective reflection wavelength λ₂ on theside of the photo-absorbing layer 14 is thus constituted.

[0100] Then, the operation of the display device according to thepresent embodiment will be explained.

[0101] In the focalconic state, the incident light passes through theliquid crystal layers 22, 26 and is absorbed by the photo-absorbinglayer 14. Accordingly, in the focalconic state, the display color isblack.

[0102] On the other hand, in the planer state, that of the incidentlight, which has wavelengths corresponding to helical pitches of theliquid crystal molecules of the liquid crystal layers 22, 26 areselectively reflected on the liquid crystal layers 22, 26. The selectivereflection wavelength λ₁ of the liquid crystal layer 22 is 495 nm, andthe selective reflection wavelength λ₂ of the liquid crystal layer 26 is601 nm. Accordingly, the display color given by mixing the reflectionlight of the liquid crystal layers 22, 26 by the additive color mixtureis white. Thus, in the planer state, the display color is white.

[0103] To change the chiral nematic liquid crystals of the liquidcrystal layers 22, 26 from the focalconic state to the planer state, acpulses of, e.g., 500 V and 100 Hz are applied between the electrodes 12,20.

[0104] To change the chiral nematic liquid crystals of the liquidcrystal layers 22, 26 from the planar state to the focalconic state, acpulses of, e.g., 200 V and 100 Hz are applied between the electrodes 12,20.

[0105] Generally, as the addition amount of the chiral catalyst islarger, the drive voltage tends to be higher. The chiral catalyst isadded in a larger amount to the liquid crystal layer 22, whose selectivereflection wavelength λ₁ is shorter than the liquid crystal layer 26,whose selective reflection wavelength λ₂, in a larger amount.Accordingly, the liquid crystal layer 26, whose selective reflectionwavelength λ₂ is long, is changed from the focalconic state to theplaner state with a lower applied voltage than in the case of the liquidcrystal layer 22, whose selective reflection wavelength λ₁ is short.Accordingly, the chiral nematic liquid crystal alone of the liquidcrystal layer 26 can be changed from the focalconic state to the planerstate by suitably setting the voltage to be applied between theelectrodes 12, 20. The voltage to be applied between the electrodes 12,20 is suitably set, whereby a display device which can display not onlyblack and while, but also chromatic colors corresponding to theselective reflection wavelength λ₂ can be provided.

[0106] (Evaluation Result)

[0107] Next, the results of the evaluation of the display deviceaccording to the present embodiment will be explained.

[0108] (a) Reflection Spectra Upon White Color Display

[0109] First, the reflection spectra upon the white color display willbe explained. FIG. 4 is a graph of the reflection spectra when thedisplay device according to the present embodiment displays white color.The wavelengths are taken on the horizontal axis, and the reflectancesare taken on the vertical axis. A D65 light source was used in measuringthe reflection spectra.

[0110] (b) Chromaticity Upon White Color Display

[0111] Next, the chromaticity upon the white color display will beexplained.

[0112] The chromaticity upon the white color display was measured, andthe result was x=0.319 and y=0.367.

[0113] Based on this, it is found that the present embodiment candisplay good white color.

[0114] (c) Display Color Changes Due to Observation Direction Changes

[0115] Next, the display color changes due to the observation directionchanges will be explained.

[0116] The display color changes due to the observation directionchanges were evaluated by changing the observation direction from 0° to60° by 10° to obtain a maximum color difference Δu′v′ in a u′v′ uniformcolor space.

[0117] As Control 1, the maximum color difference Δu′v′ was measured ona single liquid crystal layer which is arranged to display red color atan observation angle of 0°. The result was that the maximum colordifference Δu′v′ was 0.162 in Control 1.

[0118] As Control 2, the maximum color difference Δu′v′ was measured ona single liquid crystal layer which is arranged to display green colorat an observation angle of 0°. The result was that the maximum colordifference Δu′v′ was 0.146 in Control 2.

[0119] As Control 3, the maximum color difference Δu′v′ was measured ona single liquid crystal layer which is arranged to display blue color atan observation angle of 0°. The result was that the maximum colordifference Δu′v′ was 0.133 in Control 3.

[0120] As an example, the maximum color difference Δu′v′ was measured onthe display device according to the present embodiment. The result wasthat the maximum color difference Δu′v′ was 0.084.

[0121] Based on this, it can be seen that the display device accordingto the present embodiment can display colors which make no substantialchange even with observation direction changes.

[0122] The present embodiment is thus arranged so that the reflectionlight on the liquid crystal layer 22 and the reflection light on theliquid crystal layer 26 mutually have the complementary colorrelationship, whereby the display device according to the presentembodiment realize good black and white display.

[0123] Furthermore, the display device according to the presentembodiment, in which the selective reflection wavelength λ₁ of theliquid crystal layer 22 is within a range of 480-500 nm, and theselective reflection wavelength λ₂ of the liquid crystal layer 26 iswithin a range of 580-640 nm, can compensate changes of the selectivereflection wavelengths λ₁, λ₂ due to the observation angle change andmakes no substantial change in the hues of the display colors even withobservation direction changes.

[0124] According to the present embodiment, only two liquid crystallayers 22, 26 are provides, whereby good black and white display can bemake, and the display device according to the present embodiment can beinexpensive.

[0125] According to the present embodiment, chiral nematic liquidcrystals are used, whereby the display contents can be retained evenwhen the power source turned off. Accordingly, the display deviceaccording to the present embodiment can have low electric powerconsumption and have memorization ability.

[0126] [A Second Embodiment]

[0127] The display device according to a second embodiment of thepresent invention will be explained with reference to FIG. 5. The samemembers of the present embodiment as those of the display deviceaccording to the first embodiment shown in FIG. 3 are represented by thesame reference numbers not to repeat or to simplify their explanation.

[0128]FIG. 5 is a sectional view of the display device according to thepresent embodiment, which shows the structure thereof.

[0129] The display device according to the present embodiment ischaracterized mainly in that one liquid crystal layer uses a chiralnematic liquid crystal whose liquid crystal molecules are twisted right,and the other liquid crystal layer uses a chiral nematic liquid crystalwhose liquid crystal molecules are twisted left (hereinafter temporarilycalled “L liquid crystal”).

[0130] As in the first embodiment, a liquid crystal layer 22 of the Rliquid crystal is provided between a substrate 18 with an electrode 20formed on and a partition layer 16.

[0131] On the other hand, a liquid crystal layer 26 a of the L liquidcrystal, which is a chiral nematic liquid crystal whose liquid crystalmolecules are twisted left, is provided between a substrate 10 with anelectrode 14 and a photo-absorbing layer 14 formed on and the partitionlayer 16. The selective reflection wavelength λ₂ of the liquid crystallayer 26 a is set at, e.g., 601 nm. The thickness of the liquid crystallayer 26 a is, e.g., 5 μm, as is the thickness of the liquid crystallayer 26 of the display device according to the first embodiment shownin FIG. 3. The chiral nematic liquid crystal is formed by adding achiral catalyst to a nematic liquid crystal. The nematic liquid crystalcan be, e.g., E48 from Merck KGaA, as described above. The chiralcatalyst can be, e.g., S811 from Merck KGaA. This chiral catalyst has acharacteristic which induces liquid crystal molecules to twist left. Theselective reflection wavelength λ₂ of the chiral nematic liquid crystalcan be suitably set by adjusting the amount of chiral catalyst to beadded to the nematic liquid crystal.

[0132] Thus, the display device according to the present embodiment isconstituted.

[0133] The display device according to the present embodiment ischaracterized mainly in that, as described above, one liquid crystallayer 22 is formed of the R liquid crystal and the other liquid crystallayer 26 is formed of the L liquid crystal.

[0134] In the display device according to the first embodiment, theincident light is reflected on the liquid crystal layer 22 in thewavelength band where the reflection spectra of the liquid crystal layer22 and the reflection spectra of the liquid crystal layer 26 overlapeach other and is not reflected substantially on the liquid crystallayer 26. Accordingly, in the display device according to the firstembodiment, less light is reflected on the liquid crystal layer 26.

[0135] In the present embodiment, however, the liquid crystal layer 22is formed of the R liquid crystal, which reflects right circularlypolarized light, and the liquid crystal layer 26 a is formed of the Lliquid crystal, which reflect left circularly polarized light. In thepresent embodiment, even if the reflection spectra of the liquid crystallayer 22 and the reflection spectra of the liquid crystal layer 26 aoverlap each other is present, the light in the wavelength band wherethe light is reflected on the liquid crystal layer 26 a is preventedfrom being reflected on the liquid crystal layer 22. Thus, according tothe present embodiment, even when the reflection spectra of the liquidcrystal layer 22 and the reflection spectra of the liquid crystal layer26 overlap each other, the decrease of light to be reflected on theliquid crystal layer 26 a can be prevented, whereby the luminosity ofthe white display can be increased.

[0136] (Evaluation Result)

[0137] Next, the results of the evaluation of the display deviceaccording to the present embodiment will be explained.

[0138] (a) Reflection Spectra Upon the White Color Display

[0139] First, the reflection spectra upon the white color display willbe explained with reference to FIG. 6. FIG. 6 is a graph of thereflection spectra upon the white color display. In measuring thereflection spectra, a D65 light source was used as in the firstembodiment.

[0140] As shown in FIG. 6, in the present embodiment, higherreflectances were obtained in comparison with those of the reflectionspectra of the display device according to the first embodiment.

[0141] In comparing the luminosity of the white color display, thedisplay device according to the present embodiment had the luminosity ofthe white color display which was 1.4 times that of the display deviceaccording to the first embodiment.

[0142] Based on this, according to the present embodiment, theluminosity of the white color display can be higher.

[0143] (b) Chromaticity Upon White Color Display

[0144] Then, the chromaticity upon the white color display will beexplained.

[0145] The result of the chromaticity measured upon the white colordisplay is x=0.328, y=0.350.

[0146] Based on this, according to the present embodiment, it is seenthat good white color display can be obtained.

[0147] (Modification)

[0148] The display device according to one modification of the presentembodiment will be explained with reference to FIG. 7. FIG. 7 is asectional view of the display device according to the presentmodification, which shows the structure thereof.

[0149] The display device according to the present modification ischaracterized mainly in that the device includes four liquid crystallayers.

[0150] As shown in FIG. 7, partition layers 30, 32 are formed, spacedfrom each other between the substrate 18 with the electrode 20 formed onthe partition layer 16.

[0151] A liquid crystal layer 22 is formed between the substrate 18 withthe electrode 20 formed on and the partition layer 30. The liquidcrystal layer 22 is formed of the R liquid crystal. The selectivereflection wavelength λ₁ of the liquid crystal layer 22 is set at, e.g.,492 nm.

[0152] The liquid crystal layer 22 a is formed between the partitionlayer 30 and the partition layer 32. The liquid crystal layer 22 a isformed of the L liquid crystal. The selective reflection wavelength λ₁of the liquid crystal layer 22 a is set at, e.g., 492 nm.

[0153] A liquid crystal layer 26 is formed between the partition layer32 and the partition layer 16. The liquid crystal layer 26 is formed ofthe R liquid crystal. The selective reflection wavelength λ₂ of theliquid crystal layer 26 is set at, e.g., 601 nm.

[0154] A liquid crystal layer 26 a is formed between the substrate 10with the electrode 12 and the photo-absorbing layer 14 formed on and thepartition layer 16. The liquid crystal layer 26 is formed of the Rliquid crystal. The selective reflection wavelength λ₂ of the liquidcrystal layer 26 a is set at, e.g., 601 nm.

[0155] According to the present modification, liquid crystal layers 22,26 of the R liquid crystal and the liquid crystal layers 22 a, 26 a ofthe L liquid crystal are formed respectively for the selectivereflection wavelength λ₁ and the selective reflection wavelength λ₂,whereby both the right circularly polarized light and the leftcircularly polarized light can be reflected. Thus, the display deviceaccording to the present embodiment can reflect the incident light withhigher efficiency and make brighter white color display.

[0156] [A Third Embodiment]

[0157] The display device according to a third embodiment of the presentinvention will be explained with reference to FIG. 8. The same membersof the present embodiment as those of the display device according tothe first or the second embodiment shown in FIGS. 3 to 7 are representedby the same reference numbers not to repeat or to simplify theirexplanation.

[0158]FIG. 8 is a sectional view of the display device according to thepresent embodiment, which shows the structure thereof.

[0159] The display device according to the present embodiment ischaracterized mainly in that the materials of the substrates and thepartition layers are films.

[0160] As shown in FIG. 8, a substrate 10 a of film, a partition layer16 a of film, and a substrate 18 a of film are formed, opposed to eachother.

[0161] A liquid crystal layer 22 is formed between the substrate 18 awith an electrode 20 formed on and the partition layer 16 a. The liquidcrystal layer 22 is formed of the R liquid crystal. The selectivereflection wavelength λ₁ of the liquid crystal layer 22 is set at, e.g.,492 nm.

[0162] A liquid crystal layer 26 is formed between the substrate 10 awith an electrode 12 and a photo-absorbing layer 14 formed on and thepartition layer 16 a. The liquid crystal layer 26 is formed of the Rliquid crystal. The selective reflection wavelength λ₂ of the liquidcrystal layer 26 is set at, e.g., 601 nm.

[0163] In the display device according to the present embodiment, thematerials of the substrates 10 a, 18 a and the partition layer l6 a arefilms. Films generally have double refractivity. Accordingly, the simpleuse of films as the materials of the substrates 10 a, 18 a and thepartition layer 16 a cannot produce good display. In the presentembodiment, the thickness, etc. of the partition layer 16 a is suitablyset so that the ordinary rays and the extraordinary rays entering theliquid crystal layer 26 have a phase difference which is odd times λ₂/2.The thickness, etc. of the partition layer 16 a are set to satisfy suchconditions, whereby good display can be realized even in a case thatfilms having double refractivity are used.

[0164] The ordinary rays and the extraordinary rays entering the liquidcrystal layer 26 are arranged to have a phase difference which is oddtimes λ₂/2, whereby the left circularly polarized light passing throughthe liquid crystal layer 22 and entering the partition layer 16 abecomes right circularly polarized light when the former enters theliquid crystal layer 26. Accordingly, the liquid crystal layer 22reflects the right circularly polarized light, but the liquid crystallayer 26 reflects the circularly polarized light which has been leftwise when entering the partition layer 16 a. Thus, according to thepresent embodiment, even if the reflection spectra of the liquid crystallayer 22 and the reflection spectra of the liquid crystal layer 26overlap each other, the decrease of light to be reflected on the liquidcrystal layer 26 can be prevented, and bright white color display can beobtained.

[0165] As described above, according to the present embodiment, thesubstrates 10 a, 18 a and the partition layer 16 a are formed of films,whereby the display device can be used in flexibly wide applications.

[0166] According to the present embodiment, the thickness, etc. of thepartition layer 16 a are set so that the ordinary rays and extraordinaryrays entering the liquid crystal layer 26 have a phase difference whichis odd times λ₂/2, whereby the display device using even films havingdouble refractivity can realize good display.

[0167] (Modification)

[0168] Then, the display device according to one modification of thedisplay device according to the present embodiment will be explainedwith reference to FIG. 9.

[0169] The display device according to the present modification ischaracterized mainly in that the R liquid crystal and the L liquidcrystal are used in combination.

[0170] As in the display device according to the third embodiment shownin FIG. 8, the liquid crystal layer 22 is formed between the substrate18 a with the electrode 20 formed on and the partition layer 16 a. Theliquid crystal layer 22 is formed of the R liquid crystal. The selectivereflection wavelength λ₁ of the liquid crystal layer 22 is, e.g., 492nm.

[0171] As in the display device according to the third embodiment shownin FIG. 8, the liquid crystal layer 26 a is formed between the substrate10 a with the electrode 12 and the photo-absorbing layer 14 formed onand the partition layer 16 a. The liquid crystal layer 26 a is formed ofthe L liquid crystal L. The selective reflection wavelength λ₂ of theliquid crystal layer 26 a is, e.g., 601 nm.

[0172] According to the present modification, the thickness, etc. of thepartition layer 16 a is suitably set so that the ordinary rays and theextraordinary rays entering the liquid crystal layer 26 a have a phasedifference which is even times λ₂/2. The thickness, etc. of thepartition layer 16 a are set to satisfy such condition, whereby gooddisplay can be obtained even if films having double refractivity areused.

[0173] The thickness, etc. of the partition layer 16 a are set so that aphase difference between the ordinary rays and the extraordinary raysentering the liquid crystal layer 26 a is even times λ₂/2, whereby theleft circularly polarized light passing through the liquid crystal layer22 and entering the partition layer 16 a remains left circularlypolarized when entering the liquid crystal layer 26 a. Accordingly, theliquid crystal layer 22 can reflect the right circularly polarizedlight, and the liquid crystal layer 26 a can reflect the left circularlypolarized light.

[0174] Thus, according to the present modification, even if thereflection spectra of the liquid crystal layer 22 and the reflectionspectra of the liquid crystal layer 26 overlap each other, the decreaseof light to be reflected on the liquid crystal layer 26 a can beprevented, and bright white display can be obtained.

[0175] As described above, according to the present modification, thethickness, etc. of the partition layer 16 a are set so that a phasedifference between the ordinary rays and the extraordinary rays enteringthe liquid crystal layer 26 a is even times λ₂/2, whereby even in thecombination of the R liquid crystal and the L liquid crystal, gooddisplay can be realized.

[0176] [A Fourth Embodiment]

[0177] The display device according to a fourth embodiment of thepresent invention will be explained with reference to FIG. 10. The samemembers of the present embodiment as those of the display deviceaccording to the first to the third embodiments shown in FIGS. 1 to 9are represented by the same reference numbers not to repeat or tosimplify their explanation.

[0178]FIG. 10 is a sectional view of the display device according to thepresent embodiment, which shows the structure thereof.

[0179] The display device according to the present embodiment ischaracterized mainly in that a liquid crystal layer is in the form ofmicrocapsules of chiral nematic liquid crystals.

[0180] As shown in FIG. 10, a liquid crystal layer 22 in the form ofmicrocapsules and a liquid crystal layer 26 a in the form ofmicrocapsules are provided between a substrate 10 with an electrode 12and a photo-absorbing layer 14 formed on and a substrate 18 with anelectrode 20 formed on. The liquid crystal layer 22 is formed of the Rliquid crystal whose selective reflection wavelength λ₁ is, e.g., 492nm. The liquid crystal layer 26 a is formed of the L liquid crystalwhose selective reflection wavelength λ₂ is, e.g., 601 nm.

[0181] According to the present embodiment, the liquid crystal layers22, 26 a are in the form of microcapsules, which prevents without apartition layer the chiral nematic liquid crystals from mixing with eachother. According to the present embodiment, the partition layer 16 isnot required, which allows the display device to be thinner.

[0182] Furthermore, according to the present embodiment, the liquidcrystal layer 22 is formed of the R liquid crystal, and the liquidcrystal layer 26 a is formed of the L liquid crystal, whereby even ifthe reflection spectra of the liquid crystal layer 22 and the reflectionspectra of the liquid crystal layer 26 a overlap with each other, thedecrease of light to be reflected on the liquid crystal layer 22 or theliquid crystal layer 26 a can be prevented, and good white display canbe obtained.

[0183] [A Fifth Embodiment]

[0184] The display device according to a fifth embodiment of the presentinvention will be explained with reference to FIGS. 11 to 23. The samemembers of the present embodiment as those of the display deviceaccording to the first to the fourth embodiments shown in FIGS. 3 to 10are represented by the same reference numbers not to repeat or tosimplify their explanation.

[0185] First, the display device according to the present embodimentwill be explained with reference to FIG. 11. FIG. 11 is a sectional viewof the display device according to the present embodiment, which showsthe structure thereof.

[0186] An electrode 12 is formed on a substrate 10. A photoconductivelayer 34 which generates charges by the application of light is formed.A photo-absorbing layer 14 is formed on the photoconductive layer 34.Over the photo-absorbing layer 14, a partition layer 16 is formed,sandwiching a liquid crystal layer 26 a of the L liquid crystal. Overthe partition layer 16 an electrode 20 is formed, sandwiching a liquidcrystal layer 22 of the R liquid crystal. A substrate 18 is formed onthe electrode 20. The liquid crystal layers 26 a and the liquid crystallayer 22 are sealed respectively with seal compounds 28, 24.

[0187] The display device according to the present embodiment ischaracterized mainly in that the liquid crystal layer 22 and the liquidcrystal layer 26 a are substantially equal to each other in thethreshold voltage. The effect produced by making the threshold voltagesof the liquid crystal layers 22, 26 a substantially equal to each otherwill be explained with reference to FIGS. 12 to 21.

[0188]FIG. 12 is a circuit diagram of an equivalent circuit of a liquidcrystal cell. FIG. 13 is a view of relationships between an appliedpulse and a voltage applied to the liquid crystal layers. FIG. 14 isviews of relationships between the specific resistances of the liquidcrystals and voltages applied to the liquid crystals. FIG. 15 is viewsof changes of the states of the liquid crystal layers given when thethreshold voltages of the liquid crystal layers are different from eachother. FIG. 16 is views of changes of the states of the liquid crystallayers given when the threshold voltages of the liquid crystal layersare substantially equal to each other. FIG. 17 is graphs of the responsecharacteristics of the liquid crystal layers given when the thresholdvoltage difference between the liquid crystal layers is large and small.FIG. 18 is graphs of the voltage characteristics of the liquid crystallayers containing no additive. FIG. 19 is a circuit diagram used inmeasuring the partial voltage ratio of the respective liquid crystallayers. FIG. 20 is graphs of the partial voltage ratios of therespective liquid crystal layers measured with alternate pulses areapplied. FIG. 21 is a graph of the voltage response of the displaydevice according to the present embodiment.

[0189] In the display device according to the first to the fourthembodiments, two or more liquid crystal layers are driven by a pair ofelectrodes. However, when the threshold voltages of the respectiveliquid crystal layers are different from each other, a contrastsufficiently utilizing the potentials cannot be provided.

[0190] The threshold electric field strength E_(CN) of chiral nematicliquid crystal is given by

E _(CN)=(π² /P _(o))×(K22/∈₀Δ∈)^(1/2)

[0191] wherein P_(o) represents a helical pitch; K₂₂, an elasticityconstant of twists; Δ∈, a dielectric constant anisotropy; and ∈₀, avacuum dielectric constant. That is, there is a relationship that as thedielectric anisotropy is higher, the drive voltage is lower. Thedielectric anisotropy means that the dielectric constant of liquidcrystal molecules varies depending on a dielectric constant differencedue to directions of the axis of the liquid crystal molecules, i.e.,alignment states. In chiral nematic liquid crystals as well, theabove-described planer state, focalconic state and homeotropic statehave dielectric constants which are much different from each other. Inthe general liquid crystals, whose dielectric anisotropy is positive,the dielectric constant is maximum in the planer state, and minimum inthe homeotropic state.

[0192] Liquid crystals are not pure insulating films and have a propertyof passing a little current due to actions of ions, etc. generatedinside. Accordingly, in an electric circuit equivalent to a liquidcrystal cell, a liquid crystal cell can be substituted by the electriccircuit having a capacitor and a resistor connected in parallel as shownin FIG. 12.

[0193] Because of such property of liquid crystals, the voltagetransition of a liquid crystal sandwiched between electrodes is as shownin FIG. 13. The initial value V₀ shown on the left in FIG. 13corresponds to a value of a dielectric constant of the capacitor. As theinitial value V_(o) is larger, a higher voltage is required to chargethe capacitor, and it means that the dielectric constant is low. Thevalue of −Δv/Δt, V₀−Δv depends on the specific resistance value of theliquid crystal.

[0194] That is, as shown in FIG. 14A, when the specific resistance ofthe liquid crystal is relatively high, the value of Δv is small, and theholding ability of the voltage is good. On the other hand, as shown inFIG. 14B, the holding ability of the voltage is degraded as the specificresistance value of the liquid crystal is smaller, which is a barrier tothe drive. Such specific resistance decrease is affected mainly by ions,etc. present in the liquid crystal.

[0195] Accordingly, when two liquid crystal layers, which are differentin the dielectric constant anisotropy and the specific resistance, aresandwiched by a pair of electrodes, the following phenomena will takeplace when driven.

[0196] When a voltage is applied to the liquid crystal layers, more ofthe voltage is divided to that of the liquid crystal layers, which has asmaller absolute value of the dielectric constant. For example, when a20 V voltage is applied to the layer structure of a liquid crystal layer22 having a dielectric constant of 8 in the planer state and a liquidcrystal layer 26 having a dielectric constant of 12 in the planer state,as shown in FIG. 15A, a 12 V voltage is applied to the liquid crystallayer 22, and an 8 V voltage is applied to the liquid crystal layer 26.

[0197] When the applied voltage is further increased, the liquid crystallayer 22, to which more of the voltage is divided, reaches the thresholdvoltage and changes from the planer state to the homeotropic state. Theliquid crystal layer 22 has, e.g., a dielectric constant of 4 in thehomeotropic state. Accordingly, as shown in FIG. 15B, even when thevoltage to be applied to the layer structure is increased to, e.g., 32V, the voltage to be applied to the liquid crystal layer 26 is 8 V,which is lower than the threshold voltage, and only the voltage to beapplied to the liquid crystal layer 22 is increased to 24 V. Thus, whenthe dielectric constants of the liquid crystal layers are different fromeach other, it is very difficult to concurrently change states of boththe liquid crystal layers.

[0198] On the other hand, when the dielectric constants of the liquidcrystal layers 22, 26 are equal to each other, as shown in FIGS. 16A and16B, voltages to be applied to the liquid crystal layers 22, 26 aresubstantially equal to each other, and states of both the liquid crystallayers can be concurrently changed.

[0199] Accordingly, when a pair of electrodes are used, in order toconcurrently change states of a plurality of liquid crystal layers,i.e., make the threshold voltages substantially equal to each other, thedielectric constants of the respective liquid crystal layers are made asequal to each other as possible, and the specific resistances as wellare made as equal to each other as possible.

[0200]FIG. 17 is graphs of the response characteristics of the liquidcrystal layers given when the threshold voltage difference between theliquid crystal layers is large and small. As shown in FIG. 17A, when athreshold voltage difference is present between the liquid crystal layer1 and the liquid crystal layer 2, a movable range Vfc where both areturned into the focalconic state is small, but as shown in FIG. 17B,when the threshold voltages of the liquid crystal layer 1 and the liquidcrystal layer 2 are substantially equal to each other, the movable rangeVfc is wide, and the display quality and drive stability can be muchimproved.

[0201] Next, the method for controlling the threshold voltages of theliquid crystal layers will be explained by means of examples.

[0202] As a liquid crystal which reflects the right circularly polarizedlight (the R liquid crystal), a suitable amount of a chiral catalystCB15 for exciting the right helical structure is added to a liquidcrystal E48 from Merck KgaA to prepare a liquid crystal of a 492 nmdominant reflection wavelength. As a liquid crystal which reflects theleft circularly polarized light (the L liquid), a suitable amount of achiral catalyst S811 for exciting the left helical structure was addedto a liquid crystal E48 from Merck KGaA to prepare a liquid crystal of a601 nm dominant reflection wavelength.

[0203]FIG. 18 shows the voltage characteristics of the discrete liquidcrystals. FIG. 18A shows the response characteristics of the liquidcrystals changing from the planer state to the focalconic state. FIG.18B shows the response characteristics of the liquid crystals changingfrom the focalconic state to the planer state. As shown, the L liquidcrystal, which has a little lower amount ratio of the chiral catalyst,has a little lower drive voltage and oppositely the R liquid crystal,which has a higher amount ratio of the chiral catalyst, has a littlehigher drive voltage. A chiral nematic liquid crystal has a lowerspecific resistance and a higher drive voltage as the addition amount ofa chiral catalyst is larger.

[0204] Then, to electrically simulate the layer state of the liquidcrystals, as shown in FIG. 19, glass cells are serially connected, acpulses were applied, and the partial voltages of the respective liquidcrystals were investigated. The result is shown in FIG. 20. As shown inFIG. 20A, more of the voltage is divided to the R liquid crystal, whichhas a larger amount ratio of the chiral catalyst and has a low absolutedielectric constant, has a low apparent threshold voltage. Then, anabout 3% of a surfactant was added to the L liquid crystal. The resultis that, as shown in FIG. 20B, the addition of the surfactant couldreverse the relationship of the partial voltage ratio. That is, theaddition of a surfactant can much change properties of the liquidcrystal layers, such as dielectric constant and specific resistance.

[0205] Based on these results, a liquid crystal layer 26 a of thedisplay device according to the present embodiment shown in FIG. 11comprises the L liquid crystal and 1.2% of a surfactant, TN-40 (fromAsahi Denka Co., Ltd.). The film thicknesses of the respective liquidcrystal layers 22, 26 a were 3 μm. The film thickness of aphotoconductive layer 34 was 10 μm.

[0206] The voltage response characteristic of the thus-constituteddisplay device according to the present embodiment was measured. Theresult is shown in FIG. 21. The threshold voltages of the liquid crystallayer 22 and the liquid crystal layer 26 a are substantially inagreement with each other, and good response characteristics could berealized.

[0207] Next, ac pulses of 500 V and 100 Hz were applied to this displaydevice, and both liquid crystal layers 22, 26 a had the planer state. Inthe planer state, the color mixture between the two layers produced goodwhite display.

[0208] Then, with a negative image mask applied to the surface on theside of the photoconductive layer 34, dc pulses of 80 V were appliedgenerally to the device while light is being applied. The two liquidcrystal layers in regions to which the light has been applied toconcurrently changed to the focalconic state and retained the planerstate in regions to which the light has not been applied. Vivid blackand white display of high contrast could be made.

[0209] Here, the mechanism for optical writing using the above-describedphotoconductive layer 34 will be explained with reference to FIGS. 22and 23. FIG. 22 is a view explaining the structure and the operation ofthe photoconductive layer, and FIG. 23 is graphs explaining the methodof the optical writing using the photoconductive layer.

[0210] To be specific, recently the photoconductive layer 34 generallycomprises, as shown in FIG. 22, a charge generating layer (CGL) 34 awhich generates charges by light application, and a charge transferlayer (CTL) 34 b which transfers the charges generated in the chargegenerating layer (CGL) 34 a.

[0211] When a photo-energy enters the CGL 34 a, precursors of chargedcarriers, having charge moments are generated and are divided intoelectrons and holes in the presence of electric fields. The CTL 34 b isusually formed of a hole-transfer type material, and the holes transferin the CTL 34 b in the presence of electric field formed by electrifiedcharges in the surface of the photosensitizer. When an optical writingdevice combining a cholesteric liquid crystal and the photoconductivelayer 34 is driven, the electrode on the side of the reflection layer isset −, and the electrode on the side of the photoconductive layer 34 isset +.

[0212] The photoconductive layer 34 comprises an organic photosensitizer(OPC) or an inorganic material, such as amorphous silicon. The OPC issuperior to other photosensitizers in durability, processability andmass productivity, and is removably mounted on flexible media. The OPChas such a lot of merits and is a material which is presently most used.

[0213] Next, the display (reflectance) characteristic of the mediumcombining a cholesteric reflection layer and the photoconductive layerwill be explained with reference to FIG. 23.

[0214]FIG. 23A is graph of the display characteristics in the drive fromthe planer state to the focalconic state with light applied and withoutlight applied, which compare both cases with each other.

[0215] With light applied, when a voltage pulse signal exceeds athreshold voltage Vtf, the reflection layer goes on changing to thefocalconic state. When a voltage at which the reflection layer fully hasthe focalconic state is Vfc, the focalconic state goes on changing againto the planer state at voltage values exceeding the Vfc.

[0216] On the other hand, without no light applied, a threshold voltageVtf′ at which the reflection layer starts to change to the focalconicstate, and a voltage Vfc′ at which the reflection layer fully has thefocalconic state are largely rise in comparison with those with lightapplied.

[0217] Here, in comparison with the respective voltage values betweenwith light applied and without light applied, the voltage value Vfc atwhich the sufficient focalconic state is obtained with light applied isbelow the threshold voltage Vtf′ of the case without light applied. Thatis, the application of the voltage value Vfc changes the part the lighthas been applied to the focalconic state, and the part the light has notbeen applied to retains the planer state.

[0218]FIG. 23B is a graph of the display characteristics in the drivefrom the focalconic state to the planar state with light applied andwithout light applied, which compare both cases with each other.

[0219] It is assumed that with light applied to, when an applied voltageexceeds Vtp, the liquid crystal goes on changing to the planer state andhas the complete planer state when the voltage is Vp.

[0220] On the other hand, it is assumed that with no light applied to,when an applied voltage exceed Vtp′, the liquid crystal goes on changingto the planer state and has the complete planer state when the voltageis Vp′.

[0221] In this case as well, in comparison of the respective voltagevalues between with light applied and without light applied, thethreshold voltage much differs depending on whether or not light isbeing applied. For example, the general application of the voltage of Vpchanges the part where the light is applied to the planer state, but thepart without the light application retains the focalconic state.

[0222] As described above, conductivity differences taking place in thephotoconductive layer after light application make different theelectric filed strength to be applied to the liquid crystal between thepart the light has been applied to and the part the light has not beenapplied to for the same applied voltage, whereby the liquid crystal canhave different states.

[0223] Such optical writing method is described in, e.g., the Japanesepublished unexamined patent application No. Hei 09-105900, SID 96Application Digest p.59, “Reflective Display with Photoconductive Layerand Bistable, Reflective Cholesteric Mixture”, Japan Hardcopy 2000,“Electronic Paper using Cholesteric Liquid Crystal, Optical ImageWriting with Organic Photosensitizer”, etc.

[0224] As described above, according to the present embodiment, thethreshold voltages of the respective liquid crystal layers are madesubstantially equal to each other, the display quality and drivestability of the display device can be much improved.

[0225] In the present embodiment, the threshold voltages of the liquidcrystal layers are controlled by the addition of a surfactant but may becontrolled by the addition of a material other than a surfactant. Forexample, the addition of a trace of an organic solvent (acetone, ethanolor others) has the effect of controlling the threshold voltages.However, a surfactant or a solvent should not be excessively contained,because there is a risk that the excessive content of a surfactant or asolvent will induce the crystallization (deposition) or denaturation ofthe liquid crystals.

[0226] In the present embodiment, the threshold voltages of the liquidcrystals are controlled by adding a surfactant to the liquid crystals,but the threshold voltages can be controlled by other means.

[0227] For example, liquid crystals which are different in thedielectric constant anisotropy may be blended in suitable amounts tothereby make the threshold voltages equal to each other. The method ofcontrolling the threshold voltages by blending suitable amounts ofliquid crystals which are different in the dielectric constantanisotropy will be explained by means of a specific example.

[0228] As the R liquid crystal, a liquid crystal of dielectric constantanisotropy Δ∈ of 6.5 and a liquid crystal of anisotropy Δ∈ of 1.9 areblended with each other in a ratio of 1:2, and a chiral catalyst CB15for exciting the right helical structure is mixed in a suitable amount,and the dominant reflection wavelength was arranged to be 492 nm. As theL liquid crystal, a liquid crystal of dielectric anisotropy Δ∈ of 6.5and a liquid crystal of anisotropy Δ∈ of 1.9 are blended with each otherin a ratio of 3:2, and a chiral catalyst S811 for exciting the lefthelical structure is mixed in a suitable amount, and the dominantreflection wavelength was arranged to be 601 nm. The basic structure ofthe display device is as shown in FIG. 17, and the thicknesses of therespective liquid crystal layers were 3 μm.

[0229] The voltage response characteristics were measured on the thusprepared display device. A good characteristic curve having thethreshold voltages well agreed with each other was obtained.

[0230] Alternate current pulses of 500 V and 100 Hz were applied to thethus prepared display device, and both liquid crystal layers 22, 26 ahad the planer state. In the planer state, the color mixture of the twolayers produced good white display.

[0231] Then, a negative image mask is applied to the surface on the sideof the photoconductive layer 34, and while light is being applied, dcpluses of 80 V are applied generally to the device. The two liquidcrystal layers 22, 26 a in regions the light has been applied to changedto the focalconic state and retained the planer state in regions thelight has not been applied to. Vivid black and white display of highcontrast could be made.

[0232] [A Sixth Embodiment]

[0233] The display device according to a sixth embodiment of the presentinvention will be explained with reference to FIG. 24. The same membersof the present embodiment as those of the displayed device according tothe first to the third embodiments shown in FIGS. 3 to 23 arerepresented by the same reference numbers not to repeat or to simplifytheir explanation.

[0234]FIG. 24 is a sectional view of the display device according to thepresent embodiment, which shows the structure thereof.

[0235] An electrode 12 is formed on a substrate 10. A photo-absorbinglayer 14 is formed on the electrode 12. An electrode 36 is formed overthe photo-absorbing layer 14, sandwiching a liquid crystal layer 26 a ofthe L liquid crystal therebetween. A substrate 38 is formed on theelectrode 36. An electrode 40 is formed on the substrate 38. Anelectrode 20 is formed over the electrode 40, sandwiching a liquidcrystal layer 22 of the R liquid crystal therebetween. A substrate 18 isformed on the electrode 20. The liquid crystal layer 26 a and the liquidcrystal layer 22 are sealed respectively with seal compounds 28, 24.

[0236] As described above, the display device according to the presentembodiment is characterized mainly in that the liquid crystal layers 22,26 a are sandwiched respectively by pairs of the electrodes, and theliquid crystal layer 22 and the liquid crystal layer 26 a can be drivenindependently of each other. The display device is thus structured,whereby the liquid crystal layers 22, 26 a can be controlledindependently in accordance with their respective properties, and thedisplay quality and drive stability can be drastically improved.

[0237] The threshold voltages of the liquid crystal layers 22, 26 a arenot essentially equal to each other, but when in consideration of theperipheral circuits, controllability, etc., it is preferable that thethreshold voltages of the respective liquid crystals are substantiallyequal to each other, as in the display device according to the fifthembodiment.

[0238] According to the present embodiment, drive electrodes are formedfor each liquid crystal layer, whereby the display quality and drivestability can be drastically improved without considering the influenceof the partial voltages of the liquid crystal layers.

[0239] In the present embodiment, the display device according to thesecond embodiment has a pair of electrodes for each liquid crystallayer, but the display device according to the first or the thirdembodiment may have a pair of electrodes for each liquid crystal layer.

[0240] [A Seventh Embodiment]

[0241] The display device according to a seventh embodiment of thepresent invention will be explained with reference to FIGS. 25 to 31.The same members of the present embodiment as those of the displaydevice according to the first to the sixth embodiments shown in FIGS. 3to 24 are represented by the same reference numbers not to repeat or tosimplify their explanation.

[0242] In the first to the fourth embodiments, 2 layers which aremutually complementary colors are laminated one on the other between apair of electrodes to thereby display good white color when both layershave the planer state and display black color when both layer have thefocalconic state. This method can provide vivid display of highcontrast.

[0243] This method is structurally simpler than other various methods.However, this method requires 2 liquid crystal layers to be held betweena pair of electrodes, which requires various contrivances as describedin, e.g., the fifth and the sixth embodiments. Then, in the presentembodiment, a display device which is easier to drive than the displaydevice according to the first to the sixth embodiments will beexplained.

[0244] First, the liquid crystal display device according to the presentembodiment will be explained with reference to FIG. 25. FIG. 25 is asectional view of the display device according to the presentembodiment, which shows a structure thereof.

[0245] An electrode 12 is formed on a substrate 10. A photo-absorbinglayer 14 is formed on the electrode 12. An electrode 36 is formed overthe photo-absorbing layer 14 with a liquid crystal layer 26 a of the Lliquid crystal interposed therebetween. A substrate 38 is formed on theelectrode 36. An electrode 40 is formed on the substrate 38. Anelectrode 20 is formed over the electrode 40 with a liquid crystal layer22 of the R liquid crystal interposed therebetween. A substrate 18 isformed on the electrode 20. The liquid crystal layers 26 a, 22 aresealed respectively with seal compounds 28, 24.

[0246] The liquid crystal layer 22 is blue color layer having thedominant wavelength λ₁ of the reflection spectra of which is about450-480 nm. The liquid crystal layer 26 a is yellow color layer havingthe dominant wavelength λ₂of the reflection spectra of which is about470-610 nm. The full width at half maximum of the reflection band of theblue color light on the liquid crystal layer 22 is 70 nm or less.

[0247] The display device according to the present embodiment includessuch liquid crystal layers 22, 26 a and drives the liquid crystal layer26 a with the liquid crystal layer 22 fixed at the planer state or thefocalconic state, whereby, based on the additive color mixture,white-blue color display and yellow-black color display are realized.

[0248] When the liquid crystal layer 22 has the planer state, and theliquid crystal layer 26 a has the planer state, white color display ismade, and blue color display is made when the liquid crystal layer 26 ahas the focalconic state. That is, the white-blue color display can berealized. On the other hand, when the liquid crystal layer 22 has thefocalconic state, and the liquid crystal layer 26 a has the planerstate, yellow display is made, and black color display is made when theliquid crystal layer 26 a has the focalconic state. That is, theyellow-black display can be realized.

[0249] As described above, the display device according to the presentembodiment can change over the display by driving the liquid crystallayer 26 a alone. The white-blue color display and the yellow-blackdisplay can be switched by only driving the liquid crystal layer 22.Accordingly, the control of the display device can be very simple. It isnot necessary to consider relationships between the liquid crystal layer22 and the liquid crystal layer 26 a, such as threshold voltages, etc.

[0250] Then, characteristics of the white-blue color display and theyellow-black color display will be explained with reference to FIGS. 26to 31. FIG. 26 is a spectral luminous efficacy curve of the eye of man.This graph shows that colors which have the same radiation energy butwhose wavelength is about 555 nm corresponding to yellowish green colorregion is most bright to the eye of man, and the visual sensitivitydecreases from the wavelength toward the shorter-wavelength side (bluecolor side) and to the longer-wavelength side (red color side).

[0251] Here, one of the 2 liquid crystal layers forming the displaydevice is fixed at the planer state, and the other of the 2 liquidcrystal layers is driven, whereby, the white-blue color display and thewhite-yellow color display can be made. In comparing the two displays,it can be intuitively understood that the white-blue color display,which displays letters in blue, which is a cool color, can be read withless psychological stress than the white-yellow color display, whichdisplays letters in yellow, which is a warm color and has lower chroma.

[0252] This understanding based on the visibility can be explained basedon the small area third color vision abnormality, which ischaracteristic of man. The small area third color vision abnormality isthe phenomena that the sensitivity to blue colors is low in small areas,and colors on the side of short wavelengths are invisible. That is, whensmall letters are displayed in blue colors, the sensitivity of the eyeto the letters is lower, whereby larger contrasts than actually measuredcan be sensed.

[0253] Based on the above, it can be understood that the white-bluecolor display has more advantages than the white-yellow display and isnext to the white-black display in the visibility.

[0254] Then, it is theoretically explained that a narrower reflectionband of blue color can finally provide higher contrast.

[0255] A simulation of the white color display with an about 90 nm fullwidth at half maximum value of the yellow layer and a full width at halfmaximum value of the blue color layer as a parameter was made. FIG. 27shows the result of computation of the luminosity of the blue colorlayer for the reflection band (the full width at half maximum value) ofthe blue color layer as a parameter. As shown, as the reflection band ofthe blue color layer is decreased, the eyes of man feel darker. That is,it is shown that letters can be displayed thick.

[0256] The computation result shows that even with the full width athalf maximum value of the yellow color layer changed, good white colordisplay can be obtained after laminated. That is, as shown in FIG. 28,the luminosity of the white color given by the color mixture was notdecreased and was substantially constant.

[0257]FIG. 29 is a graph of the contrast of the white-blue color displayat that time. As shown, as the full width at half maximum value of theblue color layer is smaller, the blue is darker, and the contrast isbetter. Oppositely, as the full width at half maximum value of the bluecolor layer is above about 70 nm, the contrast is less than 5, and thevisibility is lower.

[0258] Based on the above, it has been shown that the reflection band ofthe yellow color layer is wider, the reflection band of the blue colorlayer is narrower, whereby good white-blue color display can beobtained. That is, in the case that only one of the liquid crystallayers is driven to switch the display, it is preferable to use the bluecolor layer as the fixed layer and the yellow color layer as the drivelayer.

[0259] On the other hand, the blue color layer of the 2 layers formingthe display device is fixed at the focalconic state, and the yellowcolor layer is used in the drive, whereby the yellow-black color displayis possible. Yellow color, which can be sensed by the vision of manamong hues at low chroma because of the visional characteristic of man,gives, upon display, less stress due to the hue than other hues, such asred, green blue, etc., and the display having good visibility.

[0260] The evaluation result of the display device according to thepresent embodiment will be explained. FIG. 30 is a graph of reflectionspectra of white color in the display device according to the presentembodiment. FIG. 31 is a graph of reflection spectra of yellow color inthe display device according to the present embodiment.

[0261] The liquid crystal layer 22 was formed of the R liquid crystalwhich was prepared by mixing a suitable amount of a chiral catalystCB15, which excites the right helical structure, in a nematic liquidcrystal of Δn=0.25 and had an about 480 nm-dominant reflectionwavelength. The full width at half maximum value of the reflection bandof the liquid crystal layer 22 was about 70 nm. The liquid crystal layer26 a was formed of the L liquid crystal which was prepared by mixing asuitable amount of a chiral catalyst S811, which excites the lefthelical structure, in a nematic liquid crystal, and had a Δn=0.33 andhad an about 590 nm-dominant reflection wavelength. The full width athalf maximum value of the reflection band of the liquid crystal layer 26a was about 105 nm. The thicknesses of the liquid crystal layer 22 andthe liquid crystal layer 26 a were respectively 3 μm.

[0262] The display device shown in FIG. 25 was formed by using thethus-adjusted liquid crystals, ac pulses of 50 V, 100 Hz were appliedrespectively between the electrode 20 and the electrode 40 and betweenthe electrode 36 and the electrode 12 to place the liquid crystal layers22, 26 a into the planer state. In this state, the reflection spectrawere measured, and the reflection spectra shown in FIG. 30 were given.x=0.318, y=0.322 and Y=0.406 are obtained, and the color mixture of the2 layers gave good white color display.

[0263] Then, ac pulses of 20V, 100 Hz were applied between the electrode36 and the electrode 12 to place the liquid crystal layer 26 a in thefocalconic state. In this state reflection spectra were measured, andgood blue color display could be given.

[0264] The contrast ratio was about 18, and although the backgroundcolor is not white, vivid images of high contrast could be given.

[0265] Then, ac pulses of 20V, 100 Hz were applied between the electrode20 and the electrode 40 to place the liquid crystal layer 22 into thefocalconic state, and ac pulses of 50 V, 100 Hz were applied between theelectrode 36 and the electrode 12 to place the liquid crystal layer intothe planer state. In this state, the reflection spectra were measured,and the reflection spectra as shown FIG. 31 were given. x=0.518, y=0.424and Y=0.324 are obtained, and bright yellow display was given.

[0266] As described above, according to the present embodiment, a bluecolor layer having the dominant wavelength λ₁ of the reflection spectraof which is about 450-480 nm is used as the first liquid crystal layer,the yellow color layer having the dominant wavelength λ₂ of thereflection spectra of which is about 470-610 nm is used as the secondliquid crystal layer, the yellow color layer alone is used as the maindrive layer, whereby the display device which can make good white colordisplay by the simple structure and the simpler drive method can berealized.

[0267] The blue color layer is driven, whereby the white-blue colordisplay and the yellow-black color display can be easily switched. Thefull width at half maximum value of the reflection band of blue light onthe blue color layer is 70 nm or below, whereby the white-blue colordisplay and the black-yellow color display of good visibility can bemade.

[0268] [An Eighth Embodiment]

[0269] The display device according to an eighth embodiment of thepresent invention will be explained with reference to FIG. 32. The samemembers of the present embodiment as those of the display deviceaccording to the first to the seventh embodiment shown in FIGS. 3 to 31are represented by the same reference numbers not repeat or to simplifytheir explanation.

[0270]FIG. 32 is a sectional view of the display device according to thepresent embodiment, which shows the structure thereof.

[0271] The display device according to the present embodiment is thesame in the basic structure as the display device according to theseventh embodiment shown in FIG. 25. The display device according to thepresent embodiment is characterized in that a photoconductive layer 34is provided between the electrode 12 and the liquid crystal layer 26 aas shown in FIG. 32. The presence of the photoconductive layer 34permits the display device according to the seventh embodiment tooptically write images.

[0272] Next, the evaluation result of the display device according tothe present embodiment will be explained.

[0273] The liquid crystal layer 22 was formed of the R liquid crystalhaving an about 480 nm-dominant reflection wavelength prepared by mixinga suitable amount of a chiral catalyst CB15, which excites the righthelical structure, in a nematic liquid crystal of Δn=0.25. The fullwidth at half maximum value of the reflection band of the liquid crystallayer 22 was about 70 nm. The liquid crystal layer 26 a was formed ofthe L liquid crystal having an about 590 nm-dominant reflectionwavelength prepared by mixing a suitable amount of a chiral catalystS811, which excites the left helical structure, in a nematic liquidcrystal of Δn=0.33. The full width at half maximum value of thereflection band of the liquid crystal layer 26 a was about 105 nm. Thethicknesses of the liquid crystal layer 22 and the liquid crystal layer26 a were respectively 3 μm. The thickness of the photoconductive layer34 was generally 10 μm.

[0274] The display device shown in FIG. 32 was formed of the thusadjusted liquid crystals, and ac pulses of 50V, 100 Hz were appliedbetween the electrode 20 and the electrode 40 to place the liquidcrystal layer 22 into the planer state.

[0275] Next, with an image mask applied to the surface on the side ofthe photoconductive layer 34, dc rectangular waves of 120 V were appliedbetween the electrode 36 and the electrode 12, whereby the liquidcrystal layer 26 a in the regions of the device where the light has beenapplied changed into the planer state, and the liquid crystal layer 26 ain the regions where the light has not been applied changed into thefocalconic state. Positive images of the white-blue color display ofgood visibility could be provided.

[0276] On the other hand, dc rectangular waves of 50 V were appliedbetween the electrode 36 and the electrode 12, whereby the liquidcrystal layer 26 a in the regions of the device where the light has beenapplied changed into the focalconic state, and the liquid crystal layer26 a in the region where the light has not been applied changed into theplaner state. Negative images of the white-blue color display of goodvisibility could be provided.

[0277] The contrast ratio was about 6.5, and the display of the level ofnewspapers, which can be seen without stress could be provided.

[0278] Then, ac pulses of 20 V, 100 Hz were applied between theelectrode 20 and the electrode 40 to place the liquid crystal layer 22into the focalconic state.

[0279] Next, with an image mask on the surface on the side of thephotoconductive layer 34, dc pulses of 100 V were between the electrode36 and the electrode 12, whereby the liquid crystal layer 26 a in theregions of the device where the light has not been applied changed intothe planer state, and the liquid crystal layer 26 a in the regions wherethe light has not been applied changed into the focalconic state.Positive images of the yellow-black color display of good visibilitycould be provided.

[0280] On the other hand, dc pulses of 50 V were applied between theelectrode 36 and the electrode 12, whereby the liquid crystal layer 26 ain the regions of the device where the light has not been appliedchanged into the focalconic state, and the liquid crystal layer 26 a inthe regions where the light has not been applied changed into the planerstate. Negative images of the yellow-black color display of goodvisibility could be provided.

[0281] The contrast ratio was about 18, and although the background wasnot white, vivid images of high contrast could be provided.

[0282] As described above, according to the present embodiment, a bluecolor layer having the dominant wavelength λ₁ of the reflection spectraof which is about 450-480 nm is used as the first liquid crystal layer,the yellow color layer having the dominant wavelength λ₂ of thereflection spectra of which is about 470-610 nm is used as the secondliquid crystal layer, and the yellow color layer alone is used as themain drive layer, whereby the display device which can make good whitecolor display by the simple structure and the simpler drive method canbe realized. The optical writing using the photoconductive layer permitsthe white-blue color display and the black-yellow color display of goodvisibility to be made.

[0283] [A Ninth Embodiment]

[0284] The display device according to a ninth embodiment of the presentinvention will be explained with reference to FIGS. 33 and 34. The samemembers of the present embodiment as those of the display deviceaccording to the first to the seventh embodiments shown in FIGS. 3 to 32are represented by the same reference numbers not to repeat or tosimplify their explanation.

[0285]FIG. 33 is a sectional view of the display device according to thepresent embodiment, which shows a structure thereof. FIG. 34 is a viewone example of a suitable ratio of partial voltages to be applied to therespective layers when a voltage is applied between the electrodes.

[0286] The display device according to the seventh and the eighthembodiments includes a pair of electrodes (the electrodes 20, 40) fordriving the liquid crystal layer 22, and a pair of electrodes (theelectrodes 12, 36) for driving the liquid crystal layer 26 a. Thedisplay device can be arranged to make the white-blue color display andthe yellow-black color display by using one pair of drive electrodes. Inthe present embodiment, such display device will be explained.

[0287] An electrode 12 is formed on a substrate 10. A photoconductivelayer 34 which generates charges by the application of light is formedon the electrode 12. A photo-absorbing layer 14 is formed on thephotoconductive layer 34. A partition layer 16 is formed over thephoto-absorbing layer 14 with a liquid crystal layer 26 a of the Lliquid crystal interposed therebetween. An electrode 20 is formed overthe partition layer 16 with a liquid crystal layer 22 of the R liquidcrystal interposed therebetween. A substrate 18 is formed on theelectrode 20. The liquid crystal layer 26 a and the liquid crystal layer22 are sealed respectively with seal compounds 28, 24.

[0288] The display device according to the present embodiment isarranged to divide as follows a voltage applied to the respective layer.

[0289]FIG. 34 show one example of the suitable ratio of partial voltagesof a voltage applied between the electrode 12 and the electrode 20divided to the respective layers. In this figure, the ratio of thethicknesses of the respective layers indicates the partial voltage ratioof the voltage.

[0290] It is preferable that as a voltage Vo to be applied to thephotoconductive layer 34, many voltages are applied to increasedisplay/non-display contrast of letters and the S/N ratio (displaynoises) . Oppositely, it is preferable that the partition layer 16 andthe photo-absorbing layer 14 are formed of materials of dielectricconstants as high as possible to thereby suppress the voltages Vd, Vkand the ratios of the partial voltages to the layers as low as possible.The voltage Vb to be applied to the liquid crystal layer 22 is madelower than the voltage Vy to be applied to the liquid crystal layer 26a. The resistivity of the liquid crystal layer 22 can be made lower byadding an additive, such as a surfactant or others, to the liquidcrystal layer 22 to thereby lower the ratio of the partial voltageapplied to the liquid crystal layer 22.

[0291] The partial voltage ratio of the partial voltages applied to therespective layers is thus controlled, whereby the liquid crystal layer26 a can be selectively driven by the voltage applied between theelectrode 12 and the electrode 30, and the display states can beswitched. The liquid crystal layer 22 can be also driven by applying ahigh voltage between the electrode 12 and the electrode 20, whereby thewhite-blue color display and the yellow-black color display can beswitched.

[0292] The liquid crystal layer 22, whose resistivity is lower,requires, for the drive, a voltage which is far higher than a drivevoltage required for printing. However, the white-blue color display andthe yellow-black color display will not be frequently switched, andpractically there is no problem.

[0293] The evaluation result of the display device according to thepresent embodiment will be explained.

[0294] The liquid crystal layer 22 was formed of the R liquid crystalhaving an about 480 nm-dominant reflection wavelength which was preparedby mixing a suitable amount of a chiral catalyst CB15, which excites theright helical structure, in a nematic liquid crystal of Δn=0.25. Asurfactant was added by about 4% to this R liquid crystal to therebymuch decrease the resistivity.

[0295] The liquid crystal layer 26 a was formed of the L liquid crystalhaving an about 590 nm-dominant reflection wavelength which was preparedby mixing a suitable amount of a chiral catalyst S811, which excites theleft helical structure, in a nematic liquid crystal of Δn=0.33.

[0296] The thicknesses of the liquid crystal layer 22 and the liquidcrystal layer 26 a were respectively 3 μm. The thickness of thephotoconductive layer 34 was generally 12 μm.

[0297] The display device shown in FIG. 33 was formed of thethus-adjusted liquid crystals, and then with an image mask applied tothe surface on the side of the photoconductive layer 34, dc pulses of500 V were applied between the electrode 12 and the electrode 20,whereby both the liquid crystal layer 22 and the liquid crystal layer 26a were initialized into the planer state.

[0298] Then, with an image mask applied to the surface on the side ofthe photoconductive layer 34, dc pulses of 150 V were applied betweenthe electrode 12 and the electrode 20, whereby the liquid crystal layer26 a regions where the light has been applied changed into thefocalconic state, and positive images of the white-blue color display ofgood visibility could be provided.

[0299] With an image mask applied to the surface on the side of thephotoconductive layer 34, dc pulses of 300 V were applied between theelectrode 12 and the electrode 20, whereby the liquid crystal layer 22and the liquid crystal layer 26 a were initialized into the focalconicstate and the planer state, respectively.

[0300] Then, with an image mask applied to the surface on the side ofthe photoconductive layer 34, dc pulses of 180 V were applied betweenthe electrode 12 and the electrode 20, whereby the liquid crystal layer26 a regions where the light has been applied changed into thefocalconic state, and positive images of the yellow-black color displayof good visibility could be provided.

[0301] As described above, in the display device according to thepresent embodiment including a first liquid crystal layer is formed of ablue color layer having an bout 450-480 nm-dominant wavelength λ₁ of thereflection spectra and a second liquid crystal layer is formed of ayellow color layer having an about 470-610 nm-dominant wavelength λ₂ ofthe reflection spectra, and using the yellow color layer alone as thedrive layer, the partial voltage ratios of voltages to be applied to thefirst liquid crystal layer and the second liquid crystal layer are muchdiffered, which permits the display device to include a pair of driveelectrodes. The display device according to the present embodiment canhave further simpler structure than the display device according to theseventh and the eighth embodiments.

[0302] [Modified Embodiments]

[0303] The present invention is not limited to the above-describedembodiments and can cover other various modifications.

[0304] For example, in the above-described embodiments, the liquidcrystal layer of a selective reflection wavelength λ₁ is disposed on theside of the observation, the liquid crystal layer of a selectivereflection wavelength λ₂ is disposed on the side of the photo-absorbinglayer 14, but it is possible that the liquid crystal layer of aselective reflection wavelength λ₂ is disposed on the side of theobservation, and the liquid crystal layer of a selective reflectionwavelength λ₁ is disposed on the side of the photo-absorbing layer 14.

[0305] In the above-described embodiments, the chiral namatic liquidcrystals of the liquid crystal layers were changed from the focalconicstate to the planer state by applying a voltage between the electrodes12, 20. However, the voltage application is not essential to change thechiral nematic liquid crystals from the focalconic state to the planerstate. For example, the application of heat, mechanical stresses orothers can change the chiral nematic liquid crystals from the focalconicstate to the planer state.

[0306] In the first and the third embodiments, all the liquid crystallayers are formed of the R liquid crystal but may be formed of the Lliquid crystal.

[0307] In the second embodiment, the modification of the thirdembodiment, and the fifth to the ninth embodiments, the liquid crystallayer of the R liquid crystal of a selective reflection wavelength λ₁ isdisposed on the side of the observation, and the liquid crystal layer ofthe L liquid crystal of a selective reflection wavelength λ₂ is disposedon the side of the photo-absorbing layer 14. However, it is possiblethat a liquid crystal layer of the L liquid crystal of a selectivewavelength λ₁ is disposed on the side of the observation, and a liquidcrystal layer of the R liquid crystal of a selective reflectionwavelength λ₂ is disposed on the side of the photo-absorbing layer 14.

[0308] In the above-described fourth embodiment, all the liquid crystallayers are micro-capsuled, but at least one of the liquid crystal layersmay be micro-capsuled. Micro-capsuling at least one of the liquidcrystal layers can prevent a plurality of the liquid crystal layers frommixing with each other.

[0309] The above-described fourth embodiment includes the liquid crystallayer of the R liquid crystal of a selective reflection wavelength λ₁and the liquid crystal layer of the L liquid crystal of a selectivereflection wavelength λ₂ but may include a liquid crystal layer of the Rliquid crystal of a selective reflection wavelength λ₁, a liquid crystallayer of the L liquid crystal of a selective reflection wavelength λ₁, aliquid crystal layer of the R liquid crystal of a selective reflectionwavelength λ₂ and a liquid crystal layer of the L liquid crystal of aselective reflection wavelength λ₂. Thus, bright white color display canbe provided.

[0310] In the fifth, the eighth and the ninth embodiments, writing isperformed by using the photoconductive layer. However, as in the displaydevice according to the first to the fourth embodiments, image displaymade performed by using the voltage alone applied between the electrode12 and the electrode 20.

[0311] In the fifth, the eighth and the ninth embodiments, thephotoconductive layer is used to form regions of one liquid crystallayer, which have different states. However, in the first to the fourthembodiments, the sixth embodiment and the seventh embodiment as well,regions of one liquid crystal layer having different states can beformed, and in this case, at least one of a pair of electrodes fordriving the liquid crystals is formed in, e.g., a matrix so as to applydrive voltages to required regions. Otherwise, the display deviceaccording to the first to the third embodiments, and the sixth and theseventh embodiments may include a photoconductive layer to displayimages by the method of optical writing.

[0312] In the fifth to the ninth embodiments, the substrates areexemplified by plate-like substrates, but films may be used as in thethird embodiment.

[0313] The above-described embodiments have been explained by means ofexamples using chiral nematic liquid crystals, but chiral nematic liquidcrystals are not essential. Liquid crystals which are able toselectively reflect incident light can be used. For example, liquidcrystals, such as cholesteric liquid crystal, etc., which can have thecholesteric phase, can be used.

[0314] In the above-described embodiments, liquid crystals are used, butliquid crystals are not essentially used. For example, electrophoreticparticles may be used, or twist balls may be used. Electrophoreticparticles and twist balls are described in, e.g., Nikkei Microdevices,February, 2001.

INDUSTRIAL APPLICABILITY

[0315] In the display device according to the present invention in whichlight reflected by first reflection means, and light reflected by asecond reflection means are mixed by additive color mixture, and thecolors are displayed, light of a first wavelength to be reflected by thefirst reflection means, and light of a second wavelength to be reflectedby the second reflection means mutually have a complementary colorrelationship, whereby good white and black display can be realized by asimple structure and a simple drive method. Accordingly, the displaydevice can usefully have low electric power consumption, andmemorization ability.

1. A display device displaying a color by mixing light reflected by afirst reflection element and light reflected by a second reflectionelement by additive color mixture, in which the light having a firstwavelength reflected by the first reflection element, and the lighthaving a second wavelength reflected by the second reflection elementhave substantially mutually complementary color relationship.
 2. Adisplay device according to claim 1, wherein the first wavelength whichis a selective reflection wavelength of the first reflection element isin one of a range of 480-500 nm and a range of 580-640 nm, and thesecond wavelength which is a selective reflection wavelength of thesecond reflection element is in the other of the range of 480-500 nm andthe range of 580-640 nm.
 3. A display device according to claim 1,wherein the first wavelength which is a selective reflection wavelengthof the first reflection element is in one of a range of 450-480 nm and arange of 470-610 nm, and the second wavelength which is a selectivereflection wavelength of the second reflection element is in the otherof the range 450-480 nm and the range of 470-610 nm.
 4. A display deviceaccording to claim 3, wherein a reflection band of that of the firstreflection element and the second reflection element, which reflectslight on a side of shorter wavelengths is narrower than a reflectionband of that of the first reflection element and the second reflectionelement which reflects light on a side of longer wavelengths.
 5. Adisplay device according to claim 4, wherein the reflection band of thereflection element which reflects light on the side of the shorterwavelengths has a full width at half maximum value of not more than 70nm.
 6. A display device according to claim 1, wherein at least one ofthe first reflection element and the second reflection element ismicro-capsuled.
 7. A display device according to claim 1, furthercomprising: a pair of electrodes for applying an electric field to thefirst reflection element and the second reflection element to change adisplay state of at least one of the first reflection element and thesecond reflection element.
 8. A display device according to claim 1,further comprising: a first pair of electrodes for applying an electricfiled to the first reflection element to change a display state of thefirst reflection element, and a second pair of electrodes for applyingan electric field to the second reflection element to change a displaystate of the second reflection element.
 9. A display device according toclaim 1, wherein a first threshold voltage for changing a display stateof the first reflection element and a second threshold voltage forchanging a display state of the second reflection element aresubstantially equal to each other.
 10. A display device according toclaim 1, further comprising: a photoconductive layer which generatescharges by an application of light, a threshold voltage differencebetween a light applied region and a light non-applied region, which hasbeen produced by the charges emitted from the photoconductive layerbeing used to selectively change a display state of the light appliedregion of the first reflection element or the second reflection element.11. A display device according to claim 1, wherein the first reflectionelement and the second reflection element are formed of liquid crystal.12. A display device according to claim 11, wherein the liquid crystalis a chiral nematic liquid crystal or a cholesteric liquid crystal. 13.A display device according to claim 1, further comprising: a partitionlayer of glass for spacing the first reflection element the secondreflection element from each other.
 14. A display device according toclaim 1, wherein the second reflection element reflects a circularlypolarized light which has a polarized direction opposite to a circularlypolarized light reflected by the first reflection element.
 15. A displaydevice according to claim 1, further comprising a partition layer of afilm sheet, for spacing the first reflection element and the secondreflection element from each other.
 16. A display device according toclaim 15, wherein the first reflection element is disposed nearer anobservation side than the second reflection element, the secondreflection element reflects a circularly polarized light which has thesame polarized direction as a circularly polarized light reflected bythe first reflection element, and a phase difference between an ordinaryray and an extraordinary ray passing through the partition layer andentering the second reflection element is substantially odd multiples ofλ/2 when a selective reflection wavelength of the second reflectionelement is λ.
 17. A display device according to claim 15, wherein thefirst reflection element is disposed nearer an observation side than thesecond reflection element, the second reflection element reflects acircularly polarized light which has a polarized direction opposite to acircularly polarized light reflected by the first reflection element,and a phase difference between an ordinary ray and an extraordinary raypassing through the partition layer and entering the second reflectionelement is substantially even multiples of λ/2 when a selectivereflection wavelength of the second reflection element is λ.
 18. Adisplay device according to claim 1, further comprising: a thirdreflection element for reflecting light at a selective reflectionwavelength which is substantially equal to that of the first reflectionelement, a fourth reflection element for reflecting light at a selectivereflection wavelength which is substantially equal to that of the secondreflection element, the third reflection element reflects a circularlypolarized light which has a polarized direction opposite to a circularlypolarized light reflected by the first reflection element, and thefourth reflection element reflects circularly polarized light which hasa polarized direction opposite to a circularly polarized light reflectedby the second reflection element.
 19. A display device according toclaim 1, wherein a reflection state of one of the first reflectionelement and the second reflection element is changed to display achromatic color.
 20. A display device according to claim 19, wherein thechromatic color is blue or yellow.
 21. A method for driving a displaydevice including a first reflection element having a selectivereflection wavelength in a 480-500 nm range and a second reflectionelement having a selective reflection wavelength in a 580-640 nm range,and displaying a color by mixing light reflected by the first reflectionelement and light reflected by the second reflection element by additivecolor mixture, in which a display state of the first reflection elementand a display state of the second reflection element being both changedto switch between a white color display and a black color display.
 22. Amethod for driving a display device including a first reflection elementhaving a selective reflection wavelength in a 450-480 nm range and asecond reflection element having a selective reflection wavelength in a570-610 nm range, and displaying a color by mixing light reflected bythe first reflection element and light reflected by the secondreflection element by additive color mixture, in which a display stateof the first reflection element being fixed, and a display state of thesecond reflection element being changed to switch between a white colordisplay and a blue color display or between a yellow color display and ablack color display.
 23. A method for driving a display device accordingto claim 22, wherein the display state of the first reflection elementis changed to switch between a white-blue color display and ayellow-black color display.